The present invention relates to a combined current differencing and operational amplifier circuit which can be fabricated as an integrated circuit and has particular, but not exclusive, application either as or in a filter in an integrated receiver.
If the output of a current differencing circuit, when considered as a separate block, is connected to an amplifier, when considered as a separate block, a problem arises due to the fact that an output offset current of the current differencing circuit does not match the input offset current of the operational amplifier. Accordingly when implementing a combined current differencing circuit and amplifier it is not desirable to simply connect the output of one part to the input of the other part.
Another aspect of implementing the combined circuit is that the current inputs to the differencing circuit may be derived from a voltage-controlled current source generally termed a transconductor. The simplest transconductor is a differential amplifier by means of which a voltage applied between the base electrodes of the different amplifier is converted into two collector signal currents of opposite phase. Normally if the transconductor is implemented using NPN transistors, then the current differencing circuit will normally be implemented using PNP transistors. As is known NPN transistors have a number of beneficial features when compared with PNP transistors in standard bipolar integrated circuit processes as they are capable of operating over a greater frequency range and have less parasitics. Accordingly it is desirable to be able to minimise the usage of PNP transistors when fabricating the current differencing circuit, especially if the input signal currents are derived from a transconductor.